Generally, an electron gun for color cathode ray tube includes: a cathode for forming the thermonic electrons into an electron beam; a triode section consisting of a first grid electrode and a second grid electrode; and a focusing electrostatic lens section for forming beam spots onto the picture screen after focusing the electron incoming from the triode section.
The electron gun described above is classified into different kinds depending on the constitution of the focusing electrostatic lens.
The electron gun for color cathode ray tube having the simplest structure is represented by a BPF (bi-potential focus) type and a UPF (uni-potential focus) type. The BPF type electron gun for color cathode ray tube includes: a triode section consisting of a cathode, a first grid electrode and a second grid electrode; and a main electrostatic focusing lens means consisting of a first accelerating and focusing electrode and a second accelerating and focusing electrode.
The UPF type electron gun for color cathode ray tube includes: a triode section constituted as mentioned above; and a main electrostatic focusing lens means consisting of a first accelerating and focusing electrode, an intermediate focusing electrode and a second accelerating and focusing electrode.
Of the electrodes of the main electrostatic focusing lens section forming a part of the BPF type electron gun for color cathode ray tube, an ultor voltage in the range of 20-30 KV is supplied to the second accelerating and focusing electrode, and a focusing voltage equivalent to 20-30% of the above mentioned ultor voltage is supplied to the first accelerating and focusing electrode.
Of the electrodes of the main electrostatic focusing lens section forming a part of the UPF type electron gun for color cathode ray tube, both the first accelerating and focusing electrode and the second accelerating and focusing electrode commonly receive the ultor voltage, and the intermediate focusing electrode receive almost a ground voltage.
These BPF and UPF type electron guns for color cathode ray tube are advantageous in that their structures are simple, thereby making it possible to attain a high assembling precesion. However, the electron beams emitted from the triode section enter into the main electrostatic focusing lens through a straight path without changing their directions, and therefore, the electron beams occupy widely to the peripheral areas of the main electrostatic focusing lens. Consequently, the difference between the focusing force around the axis of the main electrostatic focusing lens and the focusing force over the peripheral areas of the main electrostatic focusing lens, i.e., the spherical aberration of the main electrostatic focusing lens is increased, thereby making it impossible to obtain small high density electron beam spots on the pictures screen of the color cathode ray tube.
A multistep focusing type electron gun for color cathode ray tube is developed in order to overcome the above described problem. This multistep focusing type electron gun is constituted such that: an upstream auxiliary lens is disposed between the triode section and the main electrostatic focusing lens means in order to slightly change the path of the electron beams released from the triode section; and then the electron beams are let to advance along straight paths toward the main electrostatic focusing lens, thereby reducing the spherical aberration of the main electrostatic focusing lens.
The multistep focusing type electron gun for color cathode ray tube includes BUF type electron gun and a UBF type electron gun, and these electron guns also employ a BPF or UPF type electrostatic lens as the auxiliary focusing lens, while a BPF or UPF type main electrostatic focusing lens is also adopted.
Such multistep focusing type electron gun for color cathode ray tube is structured in such a manner that the characteristics of the beam spots should be improved.
Rather than the above mentioned BUF or UBF type plain multistep type electron gun, the electron gun may include the upstream auxiliary focusing lens in a multistepped form, thereby forming a multiplex multistep type focusing type electrostatic lens in a color cathode ray tube. If such a multiplex multistep focusing type electron gun is employed in a color cathode ray tube, it will not only improve the characteristics of the beam spots, but also will make it possible to obtain acceptable beams spots even on the peripheral areas of the screen.
However, if the above described multiplex multistep focusing type electron gun is to be formed, a large number of electrodes are required, and the stacked length of the electrodes, i.e., the length of the electron gun is extended, with the result that the efficiency of the assembling of the electron gun is aggravated, and that the assembling precision is deteriorated.
The assembling precision of the electron gun mentioned above mainly refers to the concentricity of the assembled electrodes, and if the electrodes of the electron gun are arranged asymmetrically around the axis of the assembled electrodes, then the electrostatic lens is asymmetrically formed around the axis. Further, the incoming electron beams will form deformed beam spots due to the astigmatism, thereby aggravating the resolving power of color cathode ray tube.